LeetCode17. Letter Combinations of a Phone Number

一、题目

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example 1:

Input: digits = “23”
Output: [“ad”,“ae”,“af”,“bd”,“be”,“bf”,“cd”,“ce”,“cf”]
Example 2:

Input: digits = “”
Output: []
Example 3:

Input: digits = “2”
Output: [“a”,“b”,“c”]

Constraints:

0 <= digits.length <= 4
digits[i] is a digit in the range [‘2’, ‘9’].

二、题解

class Solution {
public:
    vector<string> res;
    string s;
    string letterMap[10] = {
        "","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"
    };
    void backtracking(string digits,int index){
        if(index == digits.size()){
            res.push_back(s);
            return;
        }
        int digit = digits[index] - '0';
        string letter = letterMap[digit];
        for(int i = 0;i < letter.length();i++){
            s.push_back(letter[i]);
            backtracking(digits,index+1);
            s.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        if(digits.length() == 0) return res;
        backtracking(digits,0);
        return res;
    }
};